As is well known, pacemakers are implanted in patients to deliver electrical stimulation pulses to the patient's heart via one or more electrodes mounted on one or more leads. In addition to being used to stimulate the patient's heart, the electrodes are also frequently used to sense electrical activity of the heart, often in response to the stimulation pulses. It would be desirable to use the same electrode both for stimulating the patient's heart and for sensing corresponding heart activity. However, delivering a stimulation pulse through an electrode results in what is referred to as lead polarization. Lead polarization is a post-stimulation artifact that results from an accumulation of charge at the electrode/tissue interface following an electrical stimulation pulse. This lead polarization (or “after-potential”) can hinder or even prevent the accurate sensing of intrinsic cardiac electrical activity that follows soon after the stimulation pulse (e.g., the evoked response of the heart tissue caused by the stimulation pulse). In some instances, the electrode or electrodes used to deliver the stimulation pulses are not able to discriminate between the after-potential and an evoked response signal, and therefore other electrodes must be used to detect cardiac activity. This requires the provision of additional electrodes that are relatively far from the site of interest, which is disadvantageous, and also requires a lead with additional components.
In modern pacing leads the electrodes, both of the tip and ring variety, often employ a surface coating of titanium nitride (TiN). The implementation of TiN as a coating material was an improvement in the pacing industry by providing an electrode/tissue interface with increased electrode/tissue capacitance. The increase in interface capacitance is a result of the increased active surface area brought about by the fractal morphology of the sputter-coated titanium nitride material. The increase in interface capacitance, in turn, lowers the polarization artifact typically seen following the pacing pulse. This falls out from the equation describing the after-potential or polarization given by equation (1), as follows.
                                          U            C                    ⁡                      (                          t              >              T                        )                          =                              U            r                    (                      1            -                                          exp                ⁡                                  (                                                            -                      T                                                                                      R                        L                                            ⁢                                              C                        H                                                                              )                                            ⁢                              exp                ⁡                                  (                                                            -                                              (                                                  t                          -                          T                                                )                                                                                                            R                        L                                            ⁢                                              C                        H                                                                              )                                                                                        (        1        )            where:
RL: resistance of the lead
CH: Helmholtz capacitance
Ur: charge voltage
Uc: polarization value
Although the lowering of the polarization value can be achieved by increasing the lead length, which in turn increases RL (the resistance of the lead), it compromises the sensing functionality of the electrode and increases energy consumption. See Schaldach, “The Electrode-Electrolyte Interface”, Electrotherapy of the Heart, Spr-Vlg, 1992.
General disclosures of body implantable electrode constructions in which either or both of the leads and electrodes are constructed of titanium or titanium alloys are well known. Other implantable medical devices are known that include a pulse generator and associated electrodes designed to prevent leakage currents in the output circuit. This is accomplished by utilization of the electrode lead-to-tissue electrolytic interface capacitance (Helmholtz capacitance) with a highly leakage resistant layer interfaced together with and formed, for example, by titanium and titanium dioxide. Moreover, electrodes for muscle stimulation are known which feature a platinum electrode which has preferably been platinized to develop a coating of platinum black, contained in a second electrode housing of suitable electrode metal which is compatible with platinum such as titanium. Finally, the use of a platinum black coating with materials other than titanium are also known to reduce source impedance and polarization. Indeed, the use of a platinum black coating with titanium has long been known to reduce source impedance and polarization.
While advances have been made in the construction of leads that provide lower polarization values, there is still a need for leads that provide even lower polarization artifacts, particularly in the case of electrodes to be used to perform both pacing and sensing.